Porous honeycomb structures are used as filter bodies for filtering particles emitted by diesel vehicles. These filter bodies are generally made of ceramic (cordierite, silicon carbide, etc.). They may be monolithic or consist of separate units. In the latter case, the units are cemented together by means of a ceramic cement. The whole is then machined to the required section, which is generally circular or elliptical. The filter body includes a plurality of passages. It is inserted into a metal enclosure. Each passage is blocked at one or the other of its ends. There are therefore inlet passages and outlet passages. The exhaust gases are therefore constrained to pass through the lateral walls of the inlet passages into the outlet passages; thus particles and soot are deposited in the filter body.
After a certain time of use, soot accumulates in the passages of the filter body, which increases the head loss caused by the filter and degrades the performance of the engine. For this reason, the filter body must be regenerated regularly, conventionally after about 7 to 10 hours of operation, when the head loss has reached a value of approximately 150 dPa (for an engine of about 2 liters cubic capacity driving on a motorway with a filter body of approximately 4 liters).
Regeneration consists in oxidizing the soot. To this end, it is necessary to heat the soot since the temperature of the exhaust gases is of the order of 300° C. whereas the flash point temperature of the soot is more of the order of 600° C., under normal operating conditions. Despite such regeneration, combustion residues remain in the filter body. Accordingly, the head loss induced by the filter body after regeneration is always higher than that induced by the filter body before regeneration. This phenomenon of clogging continues on each regeneration and it is necessary for the dealer to clean the filter block completely, for example every 80 000 km. This cleaning constitutes a drawback to the use of filter bodies.
FR 2 473 113 proposes a filter body that can be produced by extrusion and has inlet passages of greater cross section than the outlet passages. The authors indicate a filtering area of the filter unit of 7.89 cm2/cm3 (i.e. 0.789 cm2/l) with a constant inlet passage cross section less than 12.9 mm2 and a wall thickness less than or equal to 0.7 mm.
However, the filter body described in FR 2 473 113 induces a high head loss, which means that the filter body must be regenerated frequently. It is therefore difficult to envisage the industrial use of this filter body.
There is therefore a need for a filter body having a low head loss throughout its service life and therefore necessitating less frequent cleaning. The invention aims to meet that need.